This invention relates to a method of forming a multiple of small projections on the inner surface of comparatively small-diameter catheter tubes such as those in balloon dilatation catheters and cerebrovascular treating catheters. The invention also relates to comparatively small-diameter catheter tubes having a multiple of small projections formed on the inner surface.
One of the methods commonly adopted today in the treatment of myocardial infarction and angina pectoris is percutaneous transluminal coronary angioplasy (PTCA), in which a dilatation catheter fitted with a balloon at an end is used to dilate the lesion (the area of stenosis) in a coronary artery, thereby improving the distal blood circulation. The balloon catheter used in PTCA comprises basically a shaft as the main body, a dilatation balloon fitted near the distal end of the shaft and a hub fitted at the proximal end of the shaft. The shaft as the main body has a first lumen that connects the hub to the balloon for inflating it with a pressurized fluid and a second lumen that extends beyond the hub and the balloon to the distal end of the catheter and through which a guide wire is to be passed.
Since the shaft needs at least two lumens having those capabilities, it is common practice to employ either a plastic tube having a multi-hole cross section such as a double-lumen tube or a coaxial shaft consisting of an inner and an outer tube. The inner tube of the coaxial shaft has a smooth inner surface as produced by either a common tubing method or an wire coating method.
With the recent expansion of the application of PTCA, an effort is being made to reduce the diameter of PTCA dilatation catheters. The ultimate purpose of this effort is to enable application to further distal lesion of the coronary artery or to reduce the invasion and, to this end, it is necessary to employ even finer guiding catheters. At the early stage of the development of PTCA, dilatation catheters had shaft sizes of at least 4 Fr (1.33 mm) but today most of them are 3 Fr (1 mm) and less. This is also true with guiding catheters, which are typically 9 Fr (3 mm), sometimes 6 Fr (2 mm), in size. Accordingly, the lumen of the dilatation catheter for the passage of the guide wire has become thinner and the resulting decrease in the clearance between the guide wire and the lumen has caused the problem of lower steerability.
Another class of small-diameter catheters are catheters for use in cerebrovascular embolization which is typically applied to aneurysms and arterio-venous malformation. With their distal end inserted to either the lesion in the brain or a nearby area, a liquid embolic substance such as a cyanoacrylate or a dimethyl sulfoxide solution of ethylene-vinyl alcohol copolymer, or a particulate embolic substance such as a polyvinyl alcohol granule or an embolic coil is injected into a vessel through the inserted distal end. Such catheters are also required to have a sufficient fineness to insure smooth insertion into vessels in the brain that are very thin and which have many bends and bifurcations.
As is generally known, the problem of reduced steerability of the guide wire through small-diameter catheters can be solved by reducing the contact area and, hence, the frictional resistance. It is thus anticipated that if the inner surface of the lumen of a dilation catheter for the passage of a guide wire is made sufficiently irregular to reduce the area of contact with the guide wire, the frictional resistance of the inner surface is reduced and, hence, the steerability of the guide wire is improved.
Two conventional methods for embossing the surface of a tube during extrusion molding are by cooling the lip of an extrusion die which is near the exit of resin and by performing extrusion with a profiled die.
Methods are also known that form small asperties on the inner surface of a tube. See, for example, JPB 89/16653 which teaches a method of extrusion molding a thermoplastic resin as the inner mold in the channel of a molten resin is vibrated to form asperities on the surface of the resin that is in contact with the inner mold. JPB 94/4301 teaches a method that comprises applying a coating of a silicone resin and an inorganic powder to the surface of a metal wire, baking the applied coating to form a primary film, then applying a tube forming resin, baking it, subsequently stretching the metal wire to an extent that does not exceed the yield point, thereby separating the tube from the primary film that has small asperities formed on the inner surface.
The method of embossing the tube surface by cooling the lip is applied extensively to the outer surface of tubes but not to the inner surface; from the structural viewpoint of the die, this method is very difficult to perform efficiently on small-diameter tubes and its use is limited to certain cases of inflation molding with a large die.
The other embossing technique which relies upon molding with a profiled die is only capable of forming ridges as strips and does not allow for molding by the wire coating method (i.e., a thin metal core is moved lengthwise as a resin is extruded through a small-bore die to cover the metal core successively); therefore, this method does not guarantee sufficient dimensional stability to enable the formation of a small-diameter tube.
The method disclosed in JPB 89/16653 requires large equipment to vibrate the inner mold and, in addition, controlling the stroke of vibrating the inner mold and the speed of extrusion in such a way as to form small asperities is very difficult to achieve. Furthermore, if a small-diameter tube is to be formed, the vibrating inner mold tends to cause unevenness in the wall thickness and other defects that will impair the dimensional stability of the part being molded.
The method described in JPB 94/4301 also has a problem in that the particles of inorganic powder shed off the metal wire during production to cause uneven asperties or remain within the lumen of the tube to potentially affect the patient when the catheter tube as the final product is inserted into his body. What should additionally be noted is that the tube formed by the method under consideration overlies the high points created by the particles of the inorganic powder on the surface of the metal wire; therefore, the tube produced is of such a structure that small recesses are formed in the generally flat inner surface. Since the guide wire will contact the generally flat areas of the inner surface of the tube, the frictional resistance developing on the guide wire will not be reduced by a satisfactory amount.
Under the circumstances, there are not available any catheter tubes that have small inside diameters not more than about 1 mm and that are embossed on the inner surface to such an extent that the frictional resistance on a guide wire can be reduced significantly.